JP4321501B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a laser printer, and more particularly to an apparatus having a removable belt unit.

2. Description of the Related Art Conventionally, as an image forming apparatus such as a laser printer, an image forming apparatus using an endless belt for conveying a sheet or performing intermediate transfer is known. Usually, such a belt can be attached to and detached from the apparatus main body as a belt unit so that it can be replaced after use for a predetermined period (see, for example, Patent Document 1). In such a belt unit, at least two belt support rollers including a driving roller are supported on a belt frame, and a belt is stretched between these belt support rollers. Further, the belt frame supports transfer means such as a transfer roller at a position facing the photosensitive drum disposed on the apparatus main body side with the belt interposed therebetween, and the transfer means is also supported by the belt frame. It is pressed toward the photosensitive drum side by biasing means such as a spring.
JP 2004-109267 A

  The belt unit as described above is often positioned with respect to the apparatus main body with respect to the axis of the driving roller in a state where the belt unit is mounted on the apparatus main body, thereby improving the accuracy of belt feeding. However, for example, when the plurality of transfer rollers are held by a resin belt frame and the photosensitive drum corresponding to each transfer roller is held by a metal main body frame in the apparatus main body, the temperature in the apparatus is When the temperature rises, as shown in FIG. 8, the positional relationship between the transfer rollers 100 and the photosensitive drums 101 is that at normal temperature due to the difference between the linear expansion coefficients of both frames (see FIG. 8A). May be displaced in the tensioning direction (front-rear direction) of the belt 102 (see FIG. 5B). When the relative position between the transfer roller and the photosensitive drum is shifted in this way, the transfer position of each color with respect to the sheet is shifted, so-called color shift occurs.

As a measure for preventing such color misregistration, for example, a temperature sensor for measuring the temperature in the apparatus is provided, and a so-called registration operation is performed in the apparatus to correct the color misregistration amount based on a mark printed on the belt. There is also a method of performing when the temperature is reached. However, with this method, the number of parts such as temperature sensors increases, and the cost increases, and it is necessary to stop the printing operation every time the registration operation is performed, which increases the waiting time of the user. There's a problem.
The present invention has been completed based on the above circumstances, and an object thereof is to provide an image forming apparatus capable of increasing the positioning accuracy of a transfer unit provided in a belt unit.

As means for achieving the above object, an image forming apparatus according to the invention of claim 1 includes an apparatus main body, an image carrier held by the apparatus main body, and a belt unit detachable from the apparatus main body. The belt unit includes an endless belt disposed to face the image carrier, a plurality of belt support rollers that stretch the belt, and the belt support roller. A belt frame that supports the image carrier and a transfer unit that is disposed at a position facing the image carrier across the belt, and that transfers the developer image carried on the image carrier. , upon mounting of the belt unit, said transfer means and the positioning means are provided for positioning in the stretched direction of the belt, said transfer means, the transfer roller rotatable about the roller shaft The transfer roller is assembled to the belt frame so as to be displaceable in the radial direction of the roller shaft, and the belt unit has a plate shape extending in the radial direction from an end of the roller shaft. It is characterized in that it is provided with a regulating member and a groove portion that is provided on the belt frame and can be fitted in a state where the regulating member is positioned in the axial direction of the roller shaft .

The invention according to claim 2 is characterized in that, in the invention according to claim 1 , the restricting member has a convex surface having a cross-section arc shape in contact surface with the inner wall of the groove portion.

According to a third aspect of the present invention, in the first or second aspect , the image carrier and the positioning means are held in a positioned state by holding members made of the same material provided in the apparatus main body. It has the characteristics in the place.

According to a fourth aspect of the present invention, in the apparatus according to the third aspect , the apparatus main body is provided with a metal body frame that holds both the image carrier and the positioning means in a positioned state as the holding member. It is characterized by

A fifth aspect of the present invention relates to the belt stretching direction of the transfer unit with respect to the image carrier when the temperature of the belt frame changes by 30 ° C. according to any one of the first to fourth aspects. It is characterized in that the change in position is 50 μm or less.

<Invention of Claim 1>
Since the transfer means provided in the belt unit is positioned by the positioning means provided on the apparatus main body side, the positioning accuracy of the transfer means relative to the image carrier held on the apparatus main body side can be improved.
Further, since the transfer means is constituted by a transfer roller, the belt can be driven more smoothly than when a transfer brush or a transfer blade is used as the transfer means.
In addition, the transfer roller is assembled to the belt frame in such a manner that it can be displaced without being fixed in the radial direction of the roller shaft. Therefore, when the belt unit is mounted on the mounting body, positioning is performed by positioning means on the apparatus body side. can do.
Further, the transfer roller is positioned in the axial direction while allowing the transfer roller to be displaced in the radial direction by fitting a plate-like regulating member extending in the radial direction from the end of the roller shaft into the groove of the belt frame. Can do.

<Invention of Claim 2 >
Since the contact surface of the restricting member with respect to the inner wall of the groove portion is a convex surface having an arc-shaped cross section, friction generated between the restricting member and the inner wall of the groove portion can be reduced. Accordingly, when the belt unit is mounted, the transfer roller moves smoothly in the radial direction of the roller shaft and is positioned.

<Invention of Claim 3 >
Since the image carrier and the positioning unit of the transfer unit are held in a positioning state by holding members of the same material having the same linear expansion coefficient, the relative positioning accuracy between the image carrier and the transfer unit with respect to temperature changes Can be improved.

<Invention of Claim 4 >
Since both the image carrier and the positioning means of the transfer means are positioned by the metal main body frame, highly accurate positioning is possible.

<Invention of Claim 5 >
When the temperature changes by 30 ° C., the change in position of the transfer means relative to the image carrier with respect to the belt stretching direction is 50 μm or less. Here, 50 μm is a limit that makes it impossible for the human retina to detect the displacement of the position of the image on the sheet material in a state where the standard observation distance defined by ISO 13660 is 300 mm. For example, it is required by the Japan Photographic Society and the Imaging Society Joint Publication Committee, “Fine Imaging and Hardcopy”, First Edition, Corona Co., Ltd., January 7, 1999, p527, etc.) . Normally, the temperature change of the belt frame at the time of printing is about 30 ° C. at the maximum. According to this configuration, the position of the transfer unit relative to the image carrier is shifted due to the temperature change. Image displacement can be reliably suppressed to the extent that it cannot be identified with the naked eye.

<First Embodiment>
Next, a first embodiment of the present invention will be described with reference to FIGS.
(Whole structure of laser printer)
FIG. 1 is a side sectional view showing a schematic configuration of a laser printer 1 as an image forming apparatus of the present embodiment. FIG. 2 is a side sectional view of the laser printer 1 in a state where a process cartridge 26 and a belt unit 15 are taken out. It is. In the following description, the right side in FIG.

  The laser printer 1 is a direct transfer tandem type color laser printer, and includes a substantially box-shaped main body casing 2 (corresponding to the apparatus main body of the present invention) as shown in FIG. A top cover 3 that can be opened and closed is provided on the top surface of the main casing 2. By opening the top cover 3, the process cartridge 26 and the belt unit 15 in the main casing 2 are opened as shown in FIG. 2. Exchange is possible. Further, on the upper surface of the upper surface cover 3, a paper discharge tray 5 on which paper 4 after image formation (corresponding to the sheet material of the present invention) is stacked is formed.

  A paper feed tray 7 on which paper 4 for forming an image is loaded is attached to the lower portion of the main body casing 2 so as to be drawn forward. In the paper feed tray 7, there is provided a paper pressing plate 9 that can be tilted to lift the front end side of the paper 4 by the bias of the spring 8. In addition, a pickup roller 10 and a separation pad 11 that presses against the pickup roller 10 by biasing a spring (not shown) are provided at a position above the front end of the paper feed tray 7. Further, a pair of paper feed rollers 12 is provided obliquely in front of the pickup roller 10, and a pair of registration rollers 13 is provided thereabove.

  When the uppermost sheet 4 of the sheet feeding tray 7 is pressed toward the pickup roller 10 by the sheet pressing plate 9 and is sandwiched between the pickup roller 10 and the separation pad 11 by the rotation of the pickup roller 10, 1 is reached. Separated by sheet. Then, the paper 4 sent out between the pickup roller 10 and the separation pad 11 is sent to the registration roller 13 by the paper feed roller 12. The registration roller 13 feeds the paper 4 onto the rear belt unit 15 at a predetermined timing.

  The belt unit 15 is attachable to and detachable from the main casing 2 and includes a conveyor belt 18 that is horizontally installed between a pair of belt support rollers 16 and 17 that are spaced apart from each other in the front-rear direction. The conveyor belt 18 is an endless belt made of a resin material such as polycarbonate, and is circulated and moved counterclockwise in FIG. 1 when the rear belt support roller 17 is driven to rotate, and is placed on the upper surface thereof. The paper 4 is conveyed backward. Inside the conveyor belt 18, four transfer rollers 19 arranged to face each photosensitive drum 31 (corresponding to an image carrier of the present invention) included in a process cartridge 26 described later are provided at regular intervals in the front-rear direction. Thus, the conveyance belt 18 is sandwiched between the photosensitive drums 31 and the corresponding transfer rollers 19. At the time of transfer, a transfer bias is applied between the transfer roller 19 and the photosensitive drum 31. The configuration of the belt unit 15 will be described in detail later.

  A cleaning roller 21 is provided below the belt unit 15 to remove toner, paper dust, and the like attached to the conveyance belt 18. The cleaning roller 21 has a structure in which a foam material made of silicon is provided around a metal shaft member, and is opposed to the metal electrode roller 22 provided in the belt unit 15 with the conveyance belt 18 interposed therebetween. Yes. A predetermined bias is applied between the cleaning roller 21 and the electrode roller 22 so that the toner on the transport belt 18 is electrically attracted to the cleaning roller 21 side. Further, the cleaning roller 21 is in contact with a metal recovery roller 23 that removes toner or the like adhering to the surface, and the recovery roller 23 is a blade for scraping off the toner or the like adhering to the surface. 24 abuts.

  Above the belt unit 15, four process cartridges 26 corresponding to the colors magenta, yellow, cyan, and black are detachably mounted side by side in the front-rear direction. An attached scanner unit 27 is provided. The scanner unit 27 irradiates the surface of the corresponding photosensitive drum 31 with laser light L for each color based on predetermined image data at high speed.

  The process cartridge 26 includes a frame-shaped cartridge frame 30, a photosensitive drum 31 and a scorotron charger 32 provided at a lower portion of the cartridge frame 30, and a developing cartridge 34 that is detachably attached to the cartridge frame 30. It has.

The photoconductor drum 31 includes a grounded metal drum body, and the surface layer thereof is covered with a positively chargeable photoconductive layer made of polycarbonate or the like.
The scorotron charger 32 is disposed opposite to the photosensitive drum 31 at a predetermined interval so as not to come into contact with the photosensitive drum 31 in the diagonally upper rear side of the photosensitive drum 31. The scorotron charger 32 uniformly charges the surface of the photosensitive drum 31 to positive polarity by generating corona discharge from a charging wire such as tungsten.

  The developing cartridge 34 has a substantially box shape, and a toner storage chamber 38 is provided in an upper portion thereof, and a supply roller 39, a developing roller 40, and a layer thickness regulating blade 41 are provided below the developing cartridge 34. Each toner storage chamber 38 stores positively chargeable nonmagnetic one-component toner of each color of yellow, magenta, cyan, and black as a developer. Each toner storage chamber 38 is provided with an agitator 42 for stirring the toner.

  The supply roller 39 is configured by coating a metal roller shaft with a conductive foam material, and the developing roller 40 is configured by coating the metal roller shaft with a conductive rubber material. Yes. The toner discharged from the toner storage chamber 38 is supplied to the developing roller 40 by the rotation of the supply roller 39 and is positively frictionally charged between the supply roller 39 and the developing roller 40. Further, the toner supplied onto the developing roller 40 enters between the layer thickness regulating blade 41 and the developing roller 40 with the rotation of the developing roller 40, where it is further sufficiently frictionally charged to have a constant thickness. It is carried on the developing roller 40 as a thin layer.

The surface of the photosensitive drum 31 is first uniformly charged positively by the scorotron charger 32 when rotating. Thereafter, exposure is performed by high-speed scanning of laser light from the scanner unit 27, and an electrostatic latent image corresponding to an image to be formed on the paper 4 is formed.
Next, when the developing roller 40 rotates and the positively charged toner carried on the developing roller 40 comes into contact with and faces the photosensitive drum 31, a static image formed on the surface of the photosensitive drum 31 is formed. The electric latent image is supplied. As a result, the electrostatic latent image on the photosensitive drum 31 is visualized, and a toner image by reversal development is carried on the surface of the photosensitive drum 31.

  Thereafter, the toner image carried on the surface of each photosensitive drum 31 is transferred while the paper 4 conveyed by the conveying belt 18 passes through each transfer position between the photosensitive drum 31 and the transfer roller 19. The images are sequentially transferred onto the paper 4 by a negative transfer bias applied to the roller 19. The sheet 4 having the toner image transferred thereon is then conveyed to the fixing device 43.

  The fixing device 43 is disposed behind the conveyance belt 18 in the main body casing 2. The fixing device 43 includes a heating roller 44 that is rotationally driven with a heat source such as a halogen lamp, and a pressure roller 45 that is disposed below the heating roller 44 so as to face the heating roller 44 and is driven to rotate. It has. The fixing device 43 fixes the toner image on the paper 4 by heating the paper 4 carrying the four color toner images while nipping and conveying the paper 4 by the heating roller 44 and the pressure roller 45. The heat-fixed paper 4 is transported to a paper discharge roller 47 provided on the upper portion of the main body casing 2 by a transport roller 46 disposed obliquely above and rearward of the fixing device 43, and the paper discharge roller 47 performs the above-described operation. The paper is discharged onto the paper discharge tray 5.

(Belt unit and its positioning structure)
FIG. 3 is an enlarged front sectional view of a part of the main body casing 2, FIG. 4 is a side sectional view showing a state before the belt unit 15 is mounted on the unit support portions 59 and 60, and FIG. FIG. 5 is a side sectional view showing a state after the unit 15 is mounted on the unit support portions 59 and 60.

  As shown in FIGS. 3 and 4, the belt unit 15 includes a belt frame 50 made of an insulating synthetic resin material and having a rectangular flat plate shape as a whole. The rear belt support roller 17 described above is supported at the rear end of the belt frame 50. The belt support roller 17 is connected to a gear mechanism (not shown) provided in the main casing 2 when the belt unit 15 is attached to the main casing 2, and is also provided in the main casing 2. It is driven by the power of a motor (not shown). Further, the front belt support roller 16 is supported at the front end of the belt frame 50 in a state of being urged forward by a spring 51, whereby an appropriate tension is applied to the transport belt 18. Further, the metal electrode roller 22 described above is rotatably supported on the lower surface of the belt frame 50. The electrode roller 22 is grounded when the belt unit 15 is mounted in the main casing 2.

  The belt frame 50 is provided with four groove-shaped transfer roller mounting portions 52 that extend in the left-right direction and open upward, and the transfer roller 19 is accommodated in each of them. The transfer roller 19 is formed by covering a metal roller shaft 19A with a conductive rubber member 19B, and the roller shaft 19A protrudes laterally from the left and right ends of the rubber member 19B. As shown in FIG. 4, roller shaft insertion holes 53 through which the roller shaft 19 </ b> A of the transfer roller 19 is inserted are provided on the left and right side surfaces of the belt frame 50. The roller shaft insertion hole 53 has a substantially rectangular shape, and the vertical and front-rear dimensions are larger than the outer diameter of the roller shaft 19A. In the state where the belt unit 15 is removed from the main body casing 2, The roller shaft 19 </ b> A can be displaced in the radial direction (vertical front and rear direction) within the range of the roller shaft insertion hole 53. Further, as shown in FIG. 3, a regulating member 55 is attached to the left end portion of the transfer roller 19 as viewed from the front of the roller shaft 19A. The restricting member 55 allows the roller shaft 19A to pass through a central cylindrical portion (not shown in detail), is rotatable relative to the roller shaft 19A, and cannot move relative to the axial direction. Is attached. In addition, a pair of plate-like restricting portions extending forward and backward (in the radial direction of the roller shaft 19A) from the cylindrical portion of the restricting member 55 are provided. The transfer roller mounting portion 52 of the belt frame 50 is provided with a groove portion 56 that extends in the front-rear direction and opens upward, and a plate-like restriction portion of the restriction member 55 is fitted into the groove portion 56. The plate-like restricting portion of the restricting member 55 has convex left and right side convex surfaces 55A, and the right and left convex surfaces 55A come into contact with the inner wall of the groove portion 56. As a result, the transfer roller 19 has a shaft. The roller shaft 19A is allowed to slide in the radial direction (up / down and front / rear direction), and the roller shaft 19A is allowed to tilt in the radial direction.

  On the other hand, as shown in FIG. 3, a metal body frame 58 (corresponding to the holding member of the present invention) is disposed in the body casing 2 so as to cover both the left and right sides of the process cartridge 26 and the belt unit 15. Yes. A pair of front and rear synthetic resin unit support portions 59 and 60 for supporting both end portions of the belt support rollers 16 and 17 are attached to the main body frame 58 (see FIG. 4). The rear unit support portion 60 has a substantially U-shaped cross section that opens upward, and an elastically deformable cross-section securing member 61 that protrudes inward is provided in the opening. As shown in FIG. 5, the unit support portion 60 is fitted with a bearing tool 17 </ b> A that rotatably supports the end of the belt support roller 17 on the rear side, and the bearing tool 17 </ b> A is prevented from coming off by a retaining metal fitting 61. As a result, the belt support roller 17 is held in a positioned state. In addition, the front unit support portion 59 has a horizontal surface 59A having a certain height on a part of the upper surface, and a bearing 16A for rotatably supporting the end portion of the front belt support roller 16 on the horizontal surface 59A. By placing the belt, the belt support roller 16 is supported while being positioned in the height direction. Further, the horizontal plane 59A of the front unit support portion 59 is configured to allow the belt support roller 16 to be displaced in the front-rear direction.

  Further, as shown in FIGS. 3 and 4, the main body frame 58 is provided with a pair of left and right bearing members 63 </ b> L and 63 </ b> R for supporting the roller shaft 19 </ b> A of the transfer roller 19 corresponding to each transfer roller 19. Yes. Each of the bearing members 63L and 63R includes a groove-shaped bearing groove 64 opened upward, and the roller shaft 19A is rotatably supported by fitting the end of the roller shaft 19A into the bearing groove 64 from above. Is done. A guide surface 65 for guiding the roller shaft 19 </ b> A into the bearing groove 64 is formed at the opening edge of the bearing groove 64. As shown in FIG. 3, the left bearing member 63L of the bearing members 63L and 63R as viewed from the front is made of an insulating synthetic resin material and is attached to the main body frame 58 so as to be displaceable in the vertical direction. The direction is held in a positioning state with respect to the main body frame 58. A spring 66 (corresponding to the biasing means of the present invention) is attached to the lower side of the bearing member 63L, and the bearing member 63R is biased upward by the spring 66.

  The right bearing member 63R as viewed from the front is made of a conductive synthetic resin material, and is attached to an insulating member 67 made of an insulating synthetic resin material fixed to the main body frame 58 so as to be vertically displaceable. Further, the front, rear, left and right directions are held in a positioning state with respect to the insulating member 67. A conductive spring 66 is attached to the lower side of the right bearing member 63R, and the bearing member 63R is urged upward by the spring 66. One end of an electrode plate 68 is connected to the lower end of the spring 66, and the other end of the electrode plate 68 extends to the outside of the main body frame 58, and a transfer bias applying means (inside the main body casing 2) ( (Not shown). This transfer bias applying means is electrically connected to the transfer roller 19 via an electrode plate 68, a spring 66, and a bearing member 63R, and applies a transfer bias between the transfer roller 19 and the photosensitive drum 31 during image formation. It is the structure to apply.

  On the other hand, the photosensitive drum 31 includes a drum shaft 31A extending from both ends of the drum body, as shown in FIG. Synthetic resin bearing bodies 70 are rotatably attached to the drum shaft 31A on the outer periphery of both ends of the drum shaft 31A. The main body frame 58 is provided with a pair of left and right drum positioning grooves 71 corresponding to the respective photosensitive drums 31. By fitting the bearing body 70 in each of the left and right drum positioning grooves 71, the drum shaft 31 </ b> A is rotatably supported in a positioning state with respect to the main body frame 58. The drum shaft 31A is connected to a gear mechanism (not shown) provided in the main body casing 2, and the photosensitive drum 31 is driven by the power of a motor (not shown). As described above, the transfer roller 19 and the photosensitive drum 31 are held in a state of being indirectly positioned by the metal main body frame 58, respectively, and when the temperature of the belt frame 50 changes by 30 ° C. (for example, 10 The change in position of the transfer roller 19 with respect to the photosensitive drum 31 in the front-rear direction (belt stretching direction) with respect to the photosensitive drum 31 is set to 50 μm or less.

  When exchanging the transport belt 18 or the like, as shown in FIG. 2, the belt unit 15 is taken out from the main casing 2 with the top cover 3 opened and each process cartridge 26 taken out. In order to mount the belt unit 15 in the main casing 2, as shown in FIG. 4, the belt unit 15 is lowered in a horizontal posture, and the bearing tools 17 </ b> A at both ends of the rear belt support roller 17 are attached to the unit support 60. While pushing inward, the bearings 16A at both ends of the front belt support roller 16 are placed on the unit support portion 59. Accordingly, as shown in FIG. 5, the belt unit 15 is held in a horizontal posture by the front and rear unit support portions 59 and 60. In this process, the roller shaft 19A of each transfer roller 19 is guided to the guide surface 65, and both ends thereof are fitted into the bearing grooves 64 of the bearing members 63L and 63R. Thereby, each transfer roller 19 is positioned in the front-rear direction.

  Subsequently, each process cartridge 26 is mounted above the belt unit 15. At this time, the photosensitive drum 31 is positioned with respect to the main body frame 58 by fitting the bearing bodies 70 attached to both ends of the drum shaft 31 </ b> A into the drum positioning grooves 71 of the main body frame 58. Further, since the transfer roller 19 is pressed downward against the biasing force of the spring 66 by the photosensitive drum 31, the transfer roller 19 is also positioned in the vertical direction.

(Effect of this embodiment)
As described above, according to the present embodiment, since the transfer roller 19 provided in the belt unit 15 is positioned by the bearing members 63L and 63R provided on the main body casing 2 side, the photosensitive held on the main body casing 2 side. The positioning accuracy of the transfer roller 19 with respect to the body drum 31 can be increased. As a result, occurrence of color misregistration can be prevented and high image quality can be achieved.

  Since the spring 66 that presses the transfer roller 19 toward the photosensitive drum 31 is provided on the main body casing 2 side, it is not necessary to support the pressing force of the spring 66 by the belt frame 50. Therefore, the rigidity of the belt frame 50 can be lowered compared with the case where the spring is provided in the belt unit, and the entire belt unit 15 can be downsized.

  A transfer bias is applied between the photoconductive drum 31 and the transfer roller 19 by a transfer bias applying means provided in the main casing 2, so that the toner image carried on the photoconductive drum 31 passes through the conveying belt 18. It is electrostatically transferred to the conveyed paper 4.

  Since the transfer means is constituted by the transfer roller 19, the belt can be driven more smoothly than when a transfer brush or a transfer blade is used as the transfer means.

  Since the positioning means includes bearing members 63L and 63R that rotatably support the roller shaft 19A, the transfer roller 19 can be positioned without hindering the rotation of the transfer roller 19.

  Since the transfer roller 19 is assembled to the belt frame 50 in a displaceable state without being fixed in the radial direction of the roller shaft 19A, when the belt unit 15 is attached to the main body casing 2, the transfer roller 19 is mounted on the main body casing 2 side. Positioning can be performed by positioning means (bearing members 63L, 63R).

  A plate-like regulating member 55 extending in the radial direction from the end of the roller shaft 19A is fitted into the groove portion 56 of the belt frame 50, thereby allowing the transfer roller 19 to move in the axial direction while allowing the transfer roller 19 to be displaced in the radial direction. Can be positioned.

  Since the contact surface of the restricting member 55 with the inner wall of the groove portion 56 forms a convex surface 55A having an arcuate cross section, friction generated between the restricting member 55 and the inner wall of the groove portion 56 can be reduced. Accordingly, when the belt unit 15 is mounted, the transfer roller 19 is smoothly moved and positioned in the radial direction of the roller shaft 19A.

  Since the belt frame 50 is made of synthetic resin, it can be manufactured at low cost. In addition, it is easy to insulate conductive parts such as the transfer roller 19 and the electrode roller 22.

Since the photosensitive drum 31 and the bearing members 63L and 63R for positioning the transfer roller 19 are held in a positioned state by members (main body frame 58) having the same linear expansion coefficient, the photosensitive drum 31 and the transfer roller 19 are positioned. The accuracy of relative positioning between the rollers 19 can be improved.
Further, since both the photosensitive drum 31 and the bearing members 63L and 63R are positioned by the metal main body frame 58, highly accurate positioning is possible.

  When the temperature changes by 30 ° C., the change in position of the transfer roller 19 relative to the photosensitive drum 31 in the belt stretching direction (front-rear direction) is 50 μm or less. Here, 50 μm is a limit that makes it impossible for the human retina to detect the displacement of the position of the image on the sheet material in a state where the standard observation distance defined by ISO 13660 is 300 mm. For example, it is required by the Japan Photographic Society and the Imaging Society Joint Publication Committee, “Fine Imaging and Hardcopy”, First Edition, Corona Co., Ltd., January 7, 1999, p527, etc.) . Normally, the temperature change of the belt frame 50 at the time of printing is about 30 ° C. at maximum, so according to this configuration, the sheet when the transfer roller 19 is displaced with respect to the photosensitive drum 31 due to the temperature change. 4 can be reliably suppressed to the extent that it cannot be identified with the naked eye.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to FIGS.
FIG. 6 is a side sectional view showing a schematic configuration of a laser printer 80 as an image forming apparatus of the present embodiment, and FIG. 7 is a side sectional view of the laser printer 80 in a state where the process cartridge 26 and the belt unit 81 are taken out. It is. In the following description, the right side in FIG.
The laser printer 80 of this embodiment is an intermediate transfer tandem type color laser printer using an intermediate transfer belt 86. In the following description, components having substantially the same functions as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.

  The laser printer 80 includes a belt unit 81 that can be attached to and detached from the main body casing 2. The belt unit 81 includes a belt frame 82 made of an insulating synthetic resin material and having a substantially triangular shape when viewed from the side. Belt support rollers 83, 84, and 85 are provided at the front end, rear end, and lower end of the belt frame 82, respectively, and the intermediate transfer belt 86 is stretched around these belt support rollers 83, 84, and 85. Four transfer rollers 19 are arranged on the upper surface of the belt frame 82. Also, below the belt unit 81, there is provided a secondary transfer roller 87 facing the belt support roller 85 at the lower end of the belt frame 82 and the intermediate transfer belt 86. The secondary transfer roller 87, the belt support roller 85, and the like. A secondary transfer bias is applied between them. In the laser printer 80, the toner images formed on the four photosensitive drums 31 are temporarily transferred onto the intermediate transfer belt 86 for four colors, and then the paper 4 is transferred between the secondary transfer roller 87 and the intermediate transfer belt 86. The toner image transferred to the intermediate transfer belt 86 is transferred onto the paper 4 when passing through the press contact position.

A main body frame (not shown) in the main body casing 2 is provided with a pair of front and rear unit support portions 59 and 60 for supporting the front and rear belt support rollers 83 and 84. Similarly, the main body frame is provided with a pair of left and right bearing members 63 </ b> L and 63 </ b> R for supporting the roller shaft 19 </ b> A of the transfer roller 19 corresponding to each transfer roller 19.
According to the present embodiment, the positioning accuracy of the transfer roller 19 can be improved also in the intermediate transfer type image forming apparatus.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention, and further, within the scope not departing from the gist of the invention other than the following. Various modifications can be made.
(1) In the above embodiment, the transfer roller and the photosensitive drum are held by the same component in the positioning state. However, according to the present invention, the positioning unit of the image carrier and the transfer unit are separated from each other. It is good also as a structure to hold | maintain.
(2) In the above embodiment, the transfer roller is used as the transfer unit. However, according to the present invention, a transfer brush, a transfer blade, or the like may be used as the transfer unit.

1 is a side sectional view showing a schematic configuration of a laser printer according to a first embodiment of the present invention. Side sectional view of the laser printer with the belt unit removed Front sectional view showing support structure of transfer roller and photosensitive drum Side sectional view showing a state before the belt unit is mounted on the unit support. Side sectional view showing a state after the belt unit is mounted on the unit support. Side sectional view showing a schematic configuration of a laser printer according to a second embodiment Side sectional view of the laser printer with the belt unit removed (A) At normal temperature (B) Explanatory diagram showing the positional relationship between the transfer roller and the photosensitive drum at high temperature

1,80 ... Laser printer (image forming device)
2. Body casing (device body)
4. Paper (sheet material)
15, 81 ... belt unit 16, 17, 83-85 ... belt support roller 18 ... conveyor belt (belt)
19. Transfer roller (transfer means)
19A ... roller shaft 31 ... photosensitive drum (image carrier)
50, 82 ... belt frame 55 ... regulating member 55A ... convex surface 56 ... groove 58 ... main body frame (holding member)
63L, 63R ... Bearing members (positioning means)
66 ... spring (biasing means)
86 ... Intermediate transfer belt (belt)

Claims (5)

An image forming apparatus comprising an apparatus main body, an image carrier held by the apparatus main body, and a belt unit detachable from the apparatus main body,
The belt unit is
An endless belt disposed to face the image carrier;
A plurality of belt support rollers for stretching the belt;
A belt frame that supports the belt support roller;
A transfer unit disposed at a position facing the image carrier with the belt interposed therebetween, and a transfer unit that transfers the developer image carried on the image carrier;
The apparatus main body is provided with positioning means for positioning the transfer means in the tension direction of the belt when the belt unit is mounted .
The transfer means is constituted by a transfer roller rotatable around a roller shaft, and the transfer roller is assembled to the belt frame in a state displaceable in the radial direction of the roller shaft,
The belt unit is
A plate-like restricting member extending in a radial direction from an end of the roller shaft;
An image forming apparatus comprising: a groove portion provided on the belt frame and capable of being fitted in a state where the regulating member is positioned in an axial direction of the roller shaft . 2. The image forming apparatus according to claim 1 , wherein a contact surface of the restricting member with respect to an inner wall of the groove portion is a convex surface having an arcuate cross section. The A image bearing member and said positioning means, the image forming apparatus according to claim 1 or claim 2, characterized in that it is held in a positioned state by a holding member made of the same material with each other provided in the apparatus main body . The image forming apparatus according to claim 3 , wherein the apparatus main body is provided with a metal main body frame that holds both the image carrier and the positioning unit in a positioned state as the holding member. apparatus. To any one of claims 1 to 4, wherein said position change about the belt tension direction of said transfer means to said image bearing member when the temperature changes 30 ° C. of the belt frame is 50μm or less The image forming apparatus described.

Images